A conventional spark plug comprises a cylindrical metal shell that is axially extended and includes a screw part formed at its outer circumference, a cylindrical insulator that is fitted in the metal shell and a cylindrical center electrode that is arranged in the insulator wherein the metal shell, the insulator and the center electrode are arranged so that respective axes thereof are approximately concentric in radial directions thereof. In addition, a cylindrical ground electrode is bent at its center to form a substantial L-shape and includes a base end that is welded to a leading end of the metal shell and a leading end that is opposed to a leading end of the center electrode. Meantime, a predetermined spark discharge gap is formed between the leading end of the center electrode and the leading end of the ground electrode. The spark plug having the above structure is attached to a cylinder head of an internal combustion such as engine and is used as an ignition source for mixture gas to be supplied to a combustion chamber.
As the engine has been recently complicated, it is further required to make the spark plug smaller. For example, it is required to reduce a nominal diameter of the screw part of the metal shell. For doing so, a thickness of the leading end of the metal shell to which the base end of the ground electrode is welded is made to be thinner, so that a thickness of the base end of the ground electrode should be also thinner. As a result, the problems such as wear of the ground electrode, lowering of the durability of the ground electrode due to increase in temperatures and fracture of the ground electrode due to vibrations are caused.
Accordingly, measures of enlarging the thickness of the ground electrode, compared to the prior art, and securing a sufficient sectional area have been suggested (for example, refer to Patent Document 1)
[Prior Documents]
[Patent Documents]
[Patent Document 1] JP-A No. 2003-7423